1. Field of the Invention
This invention relates to semiconductor wafer lithography systems and to prealignment and global alignment of wafers in such lithography systems.
2. Description of Related Art
During fabrication of an integrated circuit (IC), a wafer lithography system projects a pattern of light onto a photoresist layer on a semiconductor wafer. The projected light changes portions of the photoresist layer, so that developing the photoresist layer forms a mask which exposes or protects different portions of the wafer. The masked wafer is then removed to a reaction chamber where a process such as etching changes the exposed portions of the wafer. Typically, a wafer lithography system forms several masks on a wafer during IC fabrication, and the masks must be aligned with each other to form a working IC.
Initially, an alignment system aligns a wafer according to the wafer's edges so that a first mask formed on the wafer has a predictable location relative to the wafer's edges. After the first mask, masks are typically precision aligned using alignment marks formed in the first or subsequent masks. A precision alignment system identifies the alignment marks and positions the wafer relative to the alignment marks for superimposing a pattern on an already processed area. Before precision alignment, the wafer must be prealigned and globally aligned so that alignment marks are within the field of view of the precision alignment system. In a mechanical prealignment system, a precision transport arm rigidly attached to an x-y stage places a wafer on a wafer holder with a maximum deviation within the tolerance of the precision alignment system, typically about .+-.40 .mu.m. The wafer may also be banked against alignment pins on the wafer holder during prealignment. Mechanical prealignment systems tend to be slow and can reduce the wafer lithography system's processing rate.
Mechanical prealignment has disadvantages in addition to being slow. For example, banking a wafer against pins can chip the wafer's edge. Also, if prealignment of one wafer is conducted in parallel with projection on a second wafer, vibrations from the prealignment may affect the projection, and a more complex and expensive mechanical prealigner is required to reduce vibrations. Some wafer lithography systems have insufficient space to mount a precision mechanical prealigner and transport arm on an x-y stage. Accordingly, alternative devices and methods for prealignment of wafers are desired which provide fast prealignment of a wafer edge for a first exposure and fast global alignment in subsequent exposures.